Chain link

ABSTRACT

Chain link comprising two side limbs at both ends joint by curved end portions. The chain Link comprises at least one endless band of fiber material wound along the perimeter of the chain link. The fiber material follows the longitudinal direction of the limbs and the curvature of the end-portions. In a chain made of interlocking chain links of this type, all tensile loads are absorbed by the fiber material, whereas the bearing load due to interlink contact, plus the link shear and bonding stresses near the contact points are absorbed by the end portions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/EP2008/061583, filed Sep. 2, 2008, which claims the benefit ofEuropean Application No. 07116747.2, filed Sep. 19, 2007, the contentsof which is incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to a chain link and a chain, e.g. long, highstrength chains of interlocking chain links for mooring lines andtension legs used to connect, offshore platforms or other buoyant marineconstructions to the sea floor or anchor chains for yachts or otherships.

BACKGROUND OF THE INVENTION

Mooring lines and tension legs are generally made from steel link chaincables or polyester ropes having a cross sectional area of up to 750cm². In service they carry tensile loads for long periods whilesubmerged in sea water. The weight of steel in sea water is 92 percentof its weight in air. Therefore, due to the weight of the steel chains,the buoyancy of the offshore platforms fixed to the sea floor by suchchains must be larger than otherwise required so they can buoy thelines.

Transport and placement of steel mooring chains and tension legs isdifficult due to their length and weight. Typically they are transportedby ship or rail to a nearby port, and offloaded to very expensive heavylift crane vessels or special anchor handling vessels for transportationand offshore installation. If their weight and bulk could be reducedsubstantially and their ability to be lengthened and shortened readilycould be improved, then they could be assembled to a predeterminedlength and more, easily transported, handled, and more rapidly installedwith less expensive and more readily available support vessels.

It has been proposed to use ropes of polyethylene fibers, such as theDyneema® fiber of DSM. The offshore industry is already using polyesterropes for deepwater mooring applications. Such materials areapproximately neutrally buoyant in sea water. The tensile strength ofsuch materials is sufficient for long term mooring design. However,ropes have the drawback that they cannot be easily gripped, since theirouter covering gets torn off, nor can they be held in place by chainstoppers. Ropes are also sensitive to the abrasive action of mud andsand particles which may penetrate and cause wear between the ropesfibers, thereby weakening of the rope. For these reasons it is oftenpreferred to use metal chain links.

As opposed to fiber ropes, chain links can be held in place by chainstoppers. The chain stoppers can be used to secure the chain at aspecific lengthy thereby adjusting the tension and optimizing therelated station keeping performance. Typically, a chain stopper has twolatches holding the chain in place, bearing upon the shoulders of asingle link. A chain is pulled through the chain stopper until thedesired position, chain angle and chain tension is obtained. An exampleof a chain stopper is for instance disclosed in U.S. Pat. No. 7,240,633.

Under axial load, the individual chain links are subjected to all formsof primary stresses, i.e. bearing, bending, shear and tensile stresses.Near the contact points between links, the bearing load due to axialtension is transformed into complex stress patterns that result in thehighest stress in the bar at symmetric locations roughly +/−45 degreeson either side of the crown. Otherwise, for a normal steel chain link,much of the steel structure is highly underutilized. This is because theexisting manufacturing processes and machinery using forged round barstock are well embedded into the traditional chain making industry,resulting in very little advancement in the chain geometry orutilization of hybrid solutions. This is particularly the case when alink is held in a chain stopper. Due to cyclic loads, the chains arealso susceptible to fatigue failure. In addition, during transport orinstallation of the chain the individual links may be subjected to highimpact loads.

The complicated stress pattern within the individual chain links whenthe chain is under tensile load hinders a straightforward use of fibersor fiber reinforced material. In fibers, the greatest strength resultswhen the direction of the fibers is in the direction of the load.Unidirectional composite materials have relatively low shear strengthparallel to the fiber direction. Link-to-link attachments cause largestresses in the composite matrix in directions having inherently lowstrength.

U.S. Pat. No. 5,269,129 discloses a chain formed of links made offiber-reinforced resin composite material. Each link has a terminal looplocated at each axial end of a long strap. Loops, located at adjacentends of successive links, are joined by relatively short connectinglinks that overlap bushings located within each of the loops. Thebushings and connecting links are held in position at each lateral sideof the links by pins and washers. A ring surrounds each link where thestrap flares to form each terminal loop. The loops may be unitary orspaced laterally to receive within the space a unitary loop of anadjoining link aligned axial Iy with the other loop. A pin locatedwithin the loops supports washers at each lateral side of the links tomaintain the position of the links and to transfer load between thelinks. Such chains have the drawback that they have only moderate impactresistance. The links comprise a number of washers, pins and otherseparate parts resulting in an elaborate to assembling of the chain.Moreover, the strength of the chain is determined by the strength of thepins linking the chain links. The chain links are shaped ratherdifferently from the traditional interlocking toroid steel chain links,so their use requires modification of existing equipment and facilities,such as chain stoppers.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a light-weight link chainwhich overcomes these problems and combines high impact resistance andhigh tensile strength with traditional link shape so that existingfacilities, in particular chain jacks and chain stoppers, can be usedwith it.

The object of the invention has been achieved by designing a chain linkcomprising two side limbs at both ends joined by curved end portionscharacterized in that the chain link comprises at least one endless bandof fiber material wound along the perimeter of the chain link. The fibermaterial follows the longitudinal direction of the limbs and thecurvature of the end-portions. This way, all tensile loads are absorbedby the fiber material, whereas the bearing load due to interlinkcontact, plus the link shear and bending stresses near the contactpoints are absorbed by the end portions. The link can be shaped in asimilar configuration as traditional oval toroid link shapes, so thatexisting facilities such as chain stoppers can be used, or may beelongated to ultimately reduce the cost per unit length of the chain.The length to width ratio of the chain links can be made larger, e.g.,to utilize fewer end pieces, while using longer limbs, e.g., limbsinterconnected with one or more studs to form H-shaped or ladder shapedsections. Ladder shaped sections having multiple studs can be used tocontrol global torsion and/or assembly. The studs can have a smalldiameter, e.g., a diameter smaller than the diameter of the limbs. Suchextended designs can for example be used to replace steel tendonscurrently used with tension leg platforms.

The endless band of fiber material can be a band of a woven orunidirectional fiber material or combinations thereof, e.g. in differentlayers. To secure the fiber material and to give it extra strength thefiber material can be embedded in a polymeric matrix, such as an epoxyor polyester matrix. This is especially the case when the fiber materialhas been wound around the chain link more than once.

A suitable material for the end portions could be steel as used inexisting offshore mooring chains, or, alternatively, specialty metals inthe areas of high contact loads and stress, combined with othersynthetic materials in the non-load bearing structure. Under tensileload, the end-portions form contact points between the various links. Inuse these contact points are heavily subjected to wear. By making theend portions of steel, the wear resistance of steel and the shearstrength is combined with the high tensile strength of the fibers. Thecenter-section can also be made of steel, but since the mechanicaltensile stresses are carried by the fiber material, the limbs can bemade with a smaller steel cross section or of a light weight materialsuch as aluminum, or a plastic material, such as polyurethane, polyepoxyor polyester.

The end-portions—and optionally also the limbs—can be fitted with arecess along their outline in which the band of reinforcement fibermaterial is disposed. This way, the fiber material is effectivelyprotected from impact and wear loads, and suffers less from impactfatigue.

In a specific embodiment the end-portions and the limbs can be formed byseparate parts. The contact faces between those parts can for example befitted with a pin and a corresponding pin hole or a similar joint toallow the pieces to interlock.

It is possible to tit the inside contact areas of the end-portions withengineered surfaces, much like the way the human shoulder works. Theend-portions would come in two varieties: male and female. The malevariety has an extrusion, the female a recess. The male piece is able torotate and slide in the female recess which reduces the wear normallyseen between the contact areas of two chains. This way, the service lifeof the chain link can be extended.

Optionally, the limbs can be linked by a stud to form an H-shapedcenter-section. This way, the chain becomes a studlink chain, which isless likely to get tangled than a studless chain, for example when in achain locker or a bundle.

The fiber material can for example be installed in a predeterminedtension, e.g. with a tension designed to most effectively mobilize theavailable strength of the different load bearing materials consideringthe geometry, and the different ultimate strength and moduli ofelasticity.

Suitable fiber materials are for instance carbon fibers, polyethylenefibers, aramide fibers and glass fibers. Suitable polyethylene fibersare for instance the Dyneema® fibers commercially available from DSM.Suitable aramide fibers are for instance Twaron®, available from Teijin,or Kevlar®, available from DuPont.

In environments that are less extreme than offshore another embodimentof a chain link according to the present invention can be used. Withthis design, the end portions and limbs are surrounded by a sleeve ofthe fiber material. The end portions and limbs can for example be formedby a foam core comprising two mirrored shapes both forming half a sideof a chain link. This foam can be polyurethane, although any materialwould be usable. The fibers can be embedded in a polymeric matrix, e.g.an epoxy or polyester matrix. Such a design can for example beattractive for the market of pleasure yachts and the like, where chainsare generally subjected to much lower mechanical loads. The chains canbe designed to be neutrally buoyant and could be marketed based on afashionable and trendy outlook.

Such a chain link can be constructed by slipping a fiber sleeve over oneof the mirrored shapes, then, the next shape would be placed next to thefirst one and the fiber would continue to be pulled or rolled from thefirst mirrored shape over the second mirrored shape. Preferably, thesleeve is somewhat longer than the perimeter of the core, so that thefirst end of the sleeve is slipped over its other end. This constructionallows the chain to be made without any weak spots, since the fibersleeve covers both the cores and essentially creates a one piece chain.The fiber sleeve can be made out of normal or pre-impregnated carbonfiber. However, pre-impregnated carbon fiber would have the advantagethat the sleeve would not have to be treated with epoxies duringapplication, thereby simplifying the construction process.

The chain links as described can form a chain by having the curved endportion of a chain link grip around a curved end portion of an adjacentchain link. These chains are particularly useful for anchoring afloating structure, such as a ship or an offshore platform, wherein atleast the chain is used to link the floating structure, e.g., to aseabed.

The chain can for example be assembled by assembling a first chain linkhaving a recess along its outer fiber, wherein subsequently a band offiber material is wound around the chain link within its recess, then asecond link is assembled having one curved end portion gripping around acurved portion of the first link, then the second link is rotated whilea fiber supply winds a fiber material around the chain link, then thesesteps are repeated assembling further interlocking chain links until achain of a desired length is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying drawing, wherein:

FIG. 1 shows in perspective view of a first embodiment, of a cable linkaccording to the present invention;

FIG. 2A shows a plan view of a second embodiment of a cable linkaccording to the present invention;

FIG. 2B shows a cross section along the line B-B′ in FIG. 2A; FIG. 2Cshows a cross section along the line C-C′ in FIG. 2A;

FIG. 3A shows a plan view of a third embodiment of a cable linkaccording to the present invention;

FIG. 3B shows a cross section along line B-B′ in FIG. 3A;

FIG. 3C shows the body of the cable link according to FIG. 3A;

FIG. 4A shows a plan view of a third embodiment of a cable linkaccording to the present invention;

FIG. 4B shows a cross section over line B-B′ in FIG. 4A;

FIG. 4C shows a longitudinal cross section over line C-C′ in FIG. 4B.

FIG. 5 shows a chain according to the present invention;

FIG. 6: shows a device for assembling a chain according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a stud link 1 comprising a body 2 of two side limbs 3, 4 ofa light weigh plastic material, such as polyurethane. At both ends theside limbs 3, 4 are mutually joined by curved steel end portions 5, 6.The end portions 5, 6 have the shape of a circular segment, the firstend portion 5 being of a smaller curvature radius than the other endportion 6. The body 2 spans a band 7 of pre-tensioned unidirectionalreinforcement fiber material, such as carbon fibers wound along theouter perimeter of the body 2. The end portions 5, 6 and the side limbs3, 4 comprise a recess 8 extending along the outer perimeter of the body2 to receive the band of reinforcement fiber material. A crossbar orstud 9 spaces the two limbs 3, 4. The crossbar 9 and the limbs 3, 4 aremade of one single piece of a light weight plastic material, such aspolyurethane foam. The end portions are made of steel.

FIG. 2A shows a chain link 11 without a stud. The link 11 comprises abody 12 with side limbs 13, 14 and end portions 15, 16. In thisparticular embodiment the end portions are of equal size. The link 11 isshown in cross section along line B-B′ in FIG. 2B. In FIG. 2C, the link11 is shown in cross section along line C-C′. An endless band 17 offiber material is sunk in an endless recess 18 extending along the outerperimeter of the body 12. The limbs 13, 14 and the end portions 15, 16are all provided with a recess. When assembled, these recesses are inline to form the endless recess 18.

FIG. 3A shows in plan view a third embodiment of a cable link accordingto the invention. Cable link 21 comprises a body 22 (see FIG. 3B)surrounded by a sleeve 27 of a fiber material embedded in a matrix of acured polymeric resin, such as an epoxy resin. The sleeve 21 has itsouter ends joined to each other to form a closed loop. The cable linkhas two straight sides 23, 24, and two circularly curved end portions25, 26. As shown in FIG. 3C, the body 22 is made of two C-shapedsections 28, 29, each forming a curved end portion 25, 26 at both endsextended with a half section of the straight edges 23, 24. Both C-shapedsections 28, 29 have one free end provided with a projection 30 andanother free end provided with a correspondingly shaped recess 31. Thesections 28, 29 are glued together to form the body 22. The body 22 ismade of a single piece of a light weight plastic material, such asfoamed polyurethane.

FIG. 4A shows a further possible main link 41 according to theinvention, shown in FIG. 4B m cross section along line B-B′. FIG. 4Cshows the same chain link in longitudinal cress section along line C-C′in FIG. 4B. As in the embodiment of FIG. 2A, a pre-tensioned fiber band47 is sunk in a recess 48 extending over the outer perimeter of the body42. In this embodiment, one of the end portions 45 is provided with abulge 49 on the inner perimeter of its curvature. The other end portion46 is provided with a corresponding socket 50. In an assembled chain,the bulge 49 of each cable link 41 is shaped to cooperate with thesocket 50 of an adjacent cable link 41 to form a ball joint orarticulation.

FIG. 5 shows a chain 51 made of interlocking oval toroid chain links 52.Each chain link 52 comprises two side limbs 53 at both ends joint bycurved end portions 54. The curved end portion 54 of a chain link 52grip around a curved end portion 54 of an adjacent chain link 52. Eachone of the chain links 52 is provided with an endless band 55 of fibermaterial wound along the perimeter of the chain link 52. The endlessbands 55 lay sunk within a recess 56 extending along the perimeter ofthe chain link 52. The curvature of the curved end portions of thetoroid links 52 have an inner diameter corresponding to the diameter ofthe side limbs 53. Accordingly, the distance between the side limbs 53corresponds to the diameter of the side limbs 53. As a result, a chainlink 52 can only slide in one direction relative to an interlockingadjacent link 52, and the contact surface between two interlocking links52 is maximized. Optionally, the side limbs 53 can bulge inwards, sothat the movement of a link 52 relative to an interlocking adjacent link52 is restricted to two degrees of freedom of rotational movement andthe links 52 can only hinge in two directions relative to the respectiveadjacent link.

The chain links 52 in FIG. 5 are of the type as shown in FIG. 2A-2C, butthey can also be of the type shown in FIG. 1, FIGS. 3A-3C or FIGS. 4A-ACor any other suitable type of chain link according to the presentinvention.

FIG. 6 shows schematically a plan view of a device 60 for assembling achain according to the present invention. The chain is made of ovaltoroid chain links 61 comprising two side limbs 63, 64 linked at bothsides by C-shaped curved end portions 65, 66. The device 60 comprisestwo parallel supply lines 67 for the simultaneous supply of two sidelimbs 63, 64 and a curved end portion 65. A third supply line 68 extendsin a direction perpendicular to the other two and serves to supplyfurther curved end portions 66. The three supply lines 67, 68 cometogether at a platform 69 with a U-shaped opening 70, where an assembledchain link 61 is positioned in a vertical position, with its side limbs63, 64 extending horizontally. The supply lines 67 transport the sidelimbs 63, 64 until they lay at opposite sides of the vertical chain linkin the U-shaped opening 70. A curved end portion 65 follows the sidelimbs 63, 64 to be attached to these at one end, while the third supplyline 68 supplies the other curved end portion 66 which passes the openinner area of the vertical chain link 61 in the U-shaped opening 70 andis then linked to the outer ends of the present side limbs 53, 63, thusforming a new chain link 61 interlocking the vertical chain link in theU-shaped opening 70. The assembled chain link 61 on the platform 69 isthen rotated while a spinner 71 spins a fiber material 72 from a roll 73of fiber material around the chain link 61. The fiber material isreceived in a recess 74 extending along the outer fiber of the chainlink 61. After winding the fiber material, the chain is moved furtherover a distance corresponding to the length of a single chain link 61,and the newly assembled chain link 61 on the platform 59 is turned to avertical position, taking the place of the chain link positioned in theU-shaped opening 70 and the stops described above are repeated until achain of a desired length is obtained.

1. A chain comprising at least two interlocking chain links, whereineach chain link comprises at least two side limbs, each of them havingtwo ends, said side limbs at said ends being joined by curved endportions, and one endless band of reinforcement fiber material woundalong the outer perimeter of each chain link, wherein a curved endportion of one chain link grips around a curved end portion of theinterlocking other chain link, wherein the curved end portion of the onelink is provided with a bulge cooperating with a corresponding socket inthe interlocking other link, said bulge being located on an insidecontact area of the end portion of the one link and said socket beinglocated on an inside contact area of the end portion of the interlockingother link.
 2. The chain according to claim 1, wherein at least the endportions of the at least two interlocking chain links are made of steel.3. The chain according to claim 1, wherein at least the end portions ofthe at least two interlocking chain links comprise a recess along theirouter perimeter to receive and protect the band of reinforcement fibermaterial.
 4. The chain according to claim 1, wherein the side limbs ofthe at least two interlocking chain links are made of a plasticmaterial.
 5. The chain according to claim 1, wherein the fiber materialis pre-tensioned.
 6. The chain according to claim 1, wherein the fibermaterial is selected from the group of carbon fibers, polyethylenefibers, aramide fibers and glass fibers.
 7. The chain according to claim1, wherein the at least two interlocking chain links are stud linkscomprising a crossbar between the two limbs.
 8. The chain according toclaim 1, wherein the band of fiber material forms a sleeve surroundingthe limbs and end portions of the at least two interlocking chain links.9. The chain according to claim 8, wherein both the end portions andside limbs are made of a plastic material.
 10. A method formanufacturing a chain according to claim 1, wherein a first chain linkis assembled having a recess along its outer perimeter, then a fibermaterial is wound around the chain link within its recess, then a secondlink is assembled having one curved end portion gripping around a curvedend portion of the first link, then the second link is rotated while afiber supply winds a fiber material around the chain link, then thesesteps are repeated assembling further interlocking chain links until achain of a desired length is obtained.